from math import sqrt

def Area(a,b,c):
    p=(a+b+c)/2
    s=sqrt(p*(p-a)*(p-b)*(p-c))
    return s

ans=0
a = float(input())
b = float(input())
c = float(input())


try:
    if(all([a+b > c,a + c >b,b + c > a])):
        ans =  Area(a,b,c)
    else:
        raise ValueError

except ValueError:
    print("无法构成合法三角形")
else:
    print("%.2f" % ans)